Measurements of characteristics of the eye are useful in providing appropriate eye care. For example, measurements of the eye have been useful in refractive surgical procedures, such as LASIK, wherein a portion of eye tissue is removed to enhance vision of a patient. Additionally, measurements are useful in studying the eye and diagnosing eye disorders.
In refractive surgical procedures, the thickness of the cornea is a valuable parameter in determining how much tissue of the eye should be removed. The thickness of the cornea is also valuable in diagnosis, prognosis, medical & surgical procedures, and monitoring of other conditions, such as glaucoma. Devices known as pachymeters (or alternatively pachometers) are used to measure the thickness of the cornea.
Corneal haze is another characteristic measured and used in the treatment and study of eyes. Corneal haze results from an alteration of corneal transparency, which may negatively impact vision. Many eye disorders such as macular corneal dystrophy, bullous keratopathy, keratoconus, lumican deficiency (in mice), fungal infection and endothelial injury are thought to contribute to corneal haze. Additionally, haze may form after surgical procedures or injuries.
One device that measures characteristics of the eye is an ultrasonic probe. Ultrasonic probes use sound waves to obtain measurement data, typically a measurement of the thickness of a cornea at a particular point. The probes generally contact the cornea during measurement, causing discomfort to the patient. Further, the measurement process using an ultrasonic probe may require several minutes if multiple measurements are necessary to achieve accurate results. The accuracy of ultrasonic pachymetry is also dependent upon accurate knowledge of the speed of sound in corneal tissue, which may range from 1200 to 2000 meters per second in different eyes. Finally, it is difficult to assure perpendicularity of the measurement axis to the corneal surface. Lack of perpendicularity may result in erroneously large measurements.
Another device that measures characteristics of the eye is a corneal confocal microscope. A confocal microscope illuminates a small region of a substance, such as a cornea, with a collimated light source focused through an objective lens to a tiny volume of space at the focal region of the lens. A detector that is “confocal” with the focal volume detects any backscattered or reflected light from the focal region. A viewable histological image is formed by a confocal microscope. One disadvantage of confocal microscopes is that they are extremely expensive and complex. Thus, confocal microscopes are not readily available to provide measurements of an eye. In addition, confocal microscopes are big, bulky devices that may be difficult to maintain, align, and transport. The confocal microscopes also cause patient discomfort due to the fact that the lens of the microscope contacts the cornea and the light source of the microscope is bright. The measurement time can last several minutes.
In a corneal confocal microscope, signals detected by the detector form images that may be analyzed to measure the thickness or opacity of the cornea and may further be used to generate a topographical or tomographical image of the cornea. Images taken of a cornea using a confocal microscope can also provide a visual representation of corneal haze.
As one might imagine, a system of quantification of corneal haze is useful in diagnosis and in assessment of various experimental treatments or prevention protocols. Many such methods for haze quantification have been developed. Clinically, haze is evaluated by “slitlamp” examination, ranked on a scale from 0 to 4 with 0 being normal and 4 being quite severe. However, this scale is subjective and only coarsely defines the amount of corneal haze.
As a result, it is desirable to have a low cost device that can measure various characteristics of the eye, such as thickness and haze. Furthermore, a standardization of corneal haze is desirable to further aid in experimentation, reporting, and analysis of corneal haze.